1. Field of the Invention
The present invention relates to a method and an apparatus for producing a high concentration ozone water solution with a long half-life period based on a gas-liquid reaction between ozone and water.
2. Description of the Related Art
Ozone (O.sub.3) is chemically instable and changes into oxygen (O.sub.2) usually in several to several tens of seconds in the air or water. In particular, when it is in contact with bacteria, such as fungi, mould and algae, or organic substances emitting offensive odors, it changes into oxygen in a moment. Ozone presents strong oxydizing action when it changes into oxygen, and the oxydizing ability is second to fluorine among the naturally existing elements, which is 7 times the oxidizing effect and about 3000 times the oxidizing rate with respect to chlorine.
For the reason above, ozone has been utilized for sterilization, deodorization and bleaching, for example, in treating tap water, sewage or waste water of the water treatment works, super-precision dry cleaning or sterilization of clean rooms in the semiconductor industry, deodorization and sterilization of the air and filters in the air conditioning, coating or oxydizing treatment prior to adhesion process in the surface treatment technology, material tests utilizing oxidizing effect, such as deterioration tests on rubber or plastic materials or electric contact points, or sterilizing and deodorizing operative appliances or medical waste matter in the medical industry. Further, such an ozone treatment has been utilized not only in large-scale industries but over smaller businesses including the household scale.
There are two ways of utilizing ozone, one using gas-phase ozone and the other using liquid-phase ozone produced by dissolving an ozone gas in the water. In either case, the aim is the same in utilizing the strong bactericidal, deodorizing and bleaching action of ozone.
Both the gas-phase ozone and the liquid-phase ozone are utilized widely as mentioned above. In particular, as shown in Table 1, the liquid-phase ozone is used for purifying or sterilizing, deodorizing and bleaching tap water or sewage as well as used for similar treatments in the fish farming, stock raising and food processing industries.
TABLE 1 ______________________________________ (Examples of uses of liquid-phase ozone) Industrial Fields Subjects Objects ______________________________________ Treatment of Public water service Water purification plants tap water Public housing (sterilization and Home-service deodorization) drinking water Cleaning of water supply tanks Making drinking water pleasing to the palate Sewage Public sewer Bleaching, deodorization and disposal systems sterilization Dyeing factories Waste water disposal and hospitals Housing Cooling towers Water sterilization and Pools and public removal of algae baths Sterilization and deodorization Production of Cold saltwater Sterilization of water used in perishable sterilization packages and retention of foods of fishes freshness and shellfishes Upgrading of finishings Thawing of frozen Retention of freshness foods Cooking appliances and Cleaning and sterili- sterization of floors zation of vegetables Cleaning of food processing places Fish farming Water tanks for Increasing amount of oxygen and stock living fishes dissolved in water and raising Drinking water for decomposition of remaining Agricultural pig raising and feed industry poultry farming Sterilization and making Seeds, vegetables meat pleasing to the and fruits palate Sterilization and cleaning processing Medical Medical appliances Sterilization industry Dental treatment Treatment of stomatitis ______________________________________
To produce an ozone water solution, there is a well-known method in which a spherical glass filter having a 30 mm diameter and a pore size of about 45 to 50 .mu.m or a porous tubular ceramic air diffuser is attached to the bottom of an ozone-water contact reaction tank to produce an ozone water solution by introducing an ozone gas generated by an ozone generator (ozonizer) into the tank through the diffuser and diffusing the so-formed ozone gas bubbles in the water contained in the tank thereby causing a gas-liquid reaction. This method is used most generally in various industrial fields.
In this case, the amount of ozone to be dissolved from the gas bubbles into water per unit time can be calculated by equation (1). EQU Q.sub.z =K.sub.L .multidot.a(K.multidot.C.sub.G -C.sub.L) (1)
where:
Q.sub.z =amount of ozone to be dissolved in the water per unit time, PA1 K.sub.L =coefficient of total matter transfer, PA1 a=total surface area of all gas bubbles present in the water (m.sup.2), PA1 K=distribution coefficient of ozone gas to water, PA1 C.sub.G =concentration of ozone gas (g/m.sup.3), PA1 C.sub.L =concentration of ozone present in the water (g/m.sup.3).
As is seen from equation (1), when the ozone water solution is produced under specific condition, both coefficient K.sub.L of total matter transfer and distribution coefficient K of ozone gas to water are not changed. Therefore, the most important parameter in producing the ozone water solution under that condition is the total surface area a of all gas bubbles present in the water.
The total surface area a of all gas bubbles is determined by the diameter of bubbles, rising velocity of bubbles, depth of water in the reaction tank or the like factors. Thus, to increase the efficiently of ozone dissolution, it is necessary to reduce the diameter of bubbles by increasing the depth or pressure of water in the the closed reaction tank.
In case of the aforementioned conventional method of producing an ozone water solution, when the depth or pressure of water in the reaction tank is increased, the pressure of ozone gas to be sent to the spherical glass filter or air diffuser attached to the bottom of the reaction tank must be also increased. However, since the increase of ozone gas pressure leads to increase of the driving power for supplying the ozone gas, the practical depth of the reaction tank is limited to several meters or less. That is, the water depth of the reaction tank is limited in the light of cost to be needed for the driving force. Therefore, it is difficult to increase the efficiency of ozone dissolution by the way of increasing the water depth or water pressure.
In case of dissolving ozone into the water by the gas-liquid contact reaction, it is preferred to reduce the diameter of gas bubbles as small as possible in view of increasing the surface area of bubbles to be in contact with water. However, the mean diameter of gas bubbles to be generated from a usual spherical glass filter or air diffuser is about 3 mm. Therefore, it is very difficult to obtain the gas bubbles having a mean diameter less than that value by such a spherical glass filter or air diffuser. Though other bubble generators based on the principle different from that of the spherical glass filter or air diffuser have been used, the mean diameter of bubbles produced thereby is also 2 to 3 mm. Accordingly, almost all of the produced gas bubbles rise vertically and linearly in the water of the reaction tank so that only a small amount of ozone can be dissolved in the water, but almost all of ozone is dispersed in the air after rising through the water. Thus, it is difficult to produce a high concentration ozone water solution with high efficiency within a predetermined time period.
Accordingly, it has been very difficult to produce a high concentration ozone water solution by the conventional methods for both the economical and technological reasons as mentioned above. Therefore, the range of practical use or industrial effectiveness of the ozone water solution has been limited so far.
Another reason for degrading effectiveness of the ozone water solution Is the fact that the half-life period of ozone present in the water is far shorter than that of gas-phase ozone. Namely, the half-life period of ozone in the water is several tens of minutes in case of neutral water at ordinary temperature, and is further shortened (e.g., up to several tens of seconds in an extreme case) by pH and termperature of water or a small amount of organic or inorganic substance present in the water. This is because the large portion of ozone dissolved in the water is consumed for oxidizing such impurities. Therefore, in such a case, it is necessary to dissolve in advance a great amount of ozone in the water to obtain an ozone water solution having a desirable concentration.
If pure water or ultra-pure water containing theoretically almost no impurities was used as the raw material water, an ozone water solution having a desirably long half-life period would be obtained. In such a case, however, the production cost of the ozone water solution becomes extremely high in proportion to the cost of the raw material water. Therefore, except for the sterilization of medical appliances or the like uses in which a relatively high cost can be allowed to some extent, such a method is not suitable generally in view of the cost. That is, an ozone water solution of a much lower cost than that case should be required, for example, for cleaning of food processing places, cleaning and sterilization of vegetables or the like in the field of perishable food production, and sterilization and deodorization of water purification plants in the field of water supply.
To solve these problems, various studies have been done so far with respect to the half-life period of the ozone water solution. However, the method which can satisfy both the desirable half-life period (e.g., 4 to 5 hours or longer) of the ozone water solution and the condition of using a low cost raw material water to produce a practically allowable low cost ozone water solution has not been established yet.